There is increasing a great demand for high-quality plastics in these days, and a large number of polymers having various novel characteristics have been developed and put on the market. Of those, optically-anisotropic, liquid-crystalline polymers characterized by the parallel orientation of molecular chains are specifically noticed, as having high fluidity and good mechanical properties. In particular, as the polymers of those types have especially high stiffness, there is much increasing a great demand for small-sized moldings of the polymers in the field of electrical engineering and electronics and also in the field of office appliances.
Known are liquid-crystalline polymers as obtained by copolymerization of p-hydroxybenzoic acid with polyethylene terephthalate (Japanese Patent Publication (JP-B) Sho-56-18016); liquid-crystalline polymers as obtained by copolymerization of p-hydroxybenzoic acid with 4,4'-dihydroxybiphenyl, t-butylhydroquinone and terephthalic acid (Japanese Patent Application Laid-Open (JP-A) Sho-62-164719); liquid-crystalline polymers as obtained by copolymerization of p-hydroxybenzoic acid with 4,4'-dihydroxybiphenyl, isophthalic acid and terephthalic acid (JP-B Sho-57-24407, JP-A Sho-60-25046); liquid-crystalline polymers as obtained by copolymerization of p-hydroxybenzoic acid with 6-hydroxy-2-naphthoic acid (JP-A Sho-54-77691), etc.
However, as having a higher melting point than ordinary polyesters such as polyethylene terephthalate and polybutylene terephthalate, those liquid-crystalline polymers are problematic in that they are colored or thermally deteriorated to have lowered mechanical characteristics during polymerization to prepare them or during molding them. To solve this problem, a method has been proposed of adding a heat stabilizer of, for example, organic phosphorus compounds, hindered phenols and the like to the monomers being polymerized to thereby improve long-lasting heat resistance of the polymers.
It is known that liquid-crystalline polymers generally have flame retardancy, and, when exposed to direct flames, they are self-foamed to form carbide layers.
However, it has been found that typical liquid-crystalline polyesters as obtained by copolymerizing a polyester, which is derived from an alkylene glycol and a dicarboxylic acid, with an acyloxylated, aromatic carboxylic acid (for example, polymers described in JP-B Sho-56-18016) have poor flame retardancy when they are in the form of thin moldings (thickness: 0.8 mm).
As one means of imparting flame retardancy to the polymers described in JP-B Sho-56-18016, known is a method of combining the polymers with an organic bromine compound and an antimony compound (JP-A Hei-1-118567). However, this method is problematic in that the resulting polymers produce much smoke when they are fired.
Given that situation, recently, it has become strongly desired to use flame retardants containing no halogen at all, in order to overcome the drawbacks of such halogen-containing flame retardants.
As other means of making the above-mentioned polymers resistant to flames without using halogen-containing flame retardants, known are a method of copolymerizing the polymers with a phosphorus compound (JP-A Hei-3-134021); and a method of adding red phosphorus to semi-aromatic liquid-crystalline polyesters such as those comprising ethylene terephthalate units and p-hydroxybenzoic acid residue units (JP-A Hei-6-299050).
However, the heat-resisting agents described in JP-B Hei-2-51524 are not always effective for improving the heat stability of polymers and for preventing polymers from being thermally deteriorated; and the method described in JP-A Hei-3-134021 could not produce copolymers with good heat resistance. It has been found that the flame-retardant resin composition comprising a semi-aromatic polyester, which is described in JP-AHei-3-137154, is problematic not only in that its heat resistance and fluidity during molding is poor but also in that, in the UL94 test for flame retardancy, its moldings having a thickness of 0.8 mm had a degree of flame retardancy of V-0 but its moldings having a smaller thickness had poor flame retardancy.
Accordingly, the subject matter of the present invention is to obtain a resin composition and its moldings which are free from the problems noted above, which, even though being thin, still have good flame retardancy, which have good thermal characteristics including good residence stability and good dry-heat resistance during molding, and which additionally have novel characteristics including hydrolysis resistance, etc.